The present invention relates to a toroidal-type continuously variable transmission which can be used as a transmission for vehicles and various industrial machines.
A toroidal-type continuously variable transmission of a double cavity system, which is used, for example, as a transmission for a car, is structured as shown in FIGS. 15 and 16. That is, as shown in FIG. 15, inside a casing 1, an input shaft 2 is rotatably supported. On the outer periphery of the input shaft 2, a circular-pipe-shaped transmission shaft 3 is supported. In this case, the transmission shaft 3 is arranged concentrically with the input shaft 2 and can be rotated with respect to the input shaft 2.
On the near-to-two-end-portions of the transmission shaft 3, first and second input disks 4 and 5 are supported respectively through their associated ball splines 6. In this case, the first and second input disks 4 and 5 are disposed concentrically with each other, and their respective inner surfaces 4a and 5a mutually opposed to each other. In addition the first and second input disks 4 and 5 can be rotated in synchronization with each other inside the casing 1.
On the periphery of the middle portion of the transmission shaft 3, first and second output disks 7 and 8 are supported through a sleeve 9. On the outer peripheral surface of the middle portion of the sleeve 9, an output gear 10 is disposed in such a manner that they are united together as an integral body. This output gear 10 is arranged concentrically with the transmission shaft 3 and has an inside diameter larger than the outside diameter of the transmission shaft 3. Also, the output gear 10 is rotatably supported through a pair of rolling bearings 12 on a support wall 11 disposed within the casing 1.
The first and second output disks 7 and 8 are spline engaged with the two end portions of the sleeve 9. In this case, the first and second output disks 7 and 8 are disposed such that their respective inner surfaces 7a and 8a are directed in the mutually opposite directions. Therefore, the inner surfaces 4a and 7a of the first input and output disks 4 and 7 are opposed to each other, while the outer surfaces 5a and 8a of the second input and output disks 5 and 8 are opposed to each other.
As shown in FIG. 16, laterally of the first and second output disks 7 and 8 inside the casing 1, a pair of yokes 13a and 13b are supported in such a manner that they sandwich the two disks 7 and 8 from both sides thereof. The pair of yokes 13a and 13b are formed of metal such as steel by press working or by forging in a rectangular shape. In addition, in order to support pivot shafts 16 respectively disposed on the respective two end portions of two trunnions 14 (which will be discussed later) in such a manner that they can be swung, there are formed circular-shaped support holes 18 in the four corners of the yokes 13a and 13b. In the width-direction central portions of the yokes 13a and 13b, there are formed circular-shaped securing holes 19.
The pair of yokes 13a and 13b are supported on a pair of spherical-surface-shaped support posts 20a and 20b respectively formed in the mutually opposed portions of the inner surface of the casing 1 in such a manner that they can be shifted slightly. The two support posts 20a and 20b are respectively disposed in a first cavity 21 and a second cavity 22 in such a manner that they are opposed to each other. The first cavity 21 is an intermediate portion between the inner surface 4a of the first input disk 4 and the inner surface 7a of the first output disk 7. The second cavity 22 is an intermediate portion between the inner surface 5a of the second input disk 5 and the inner surface 8a of the second output disk 8. Therefore, in the state of that the pair of yokes 13a and 13b are supported on their associated support posts 20a and 20b, they are disposed in such a manner that the one-end portions of the pair of yokes 13a and 13b are opposed to the outer peripheral portion of the first cavity 21, and the other-end portions thereof are opposed to the outer peripheral portion of the second cavity 22 in an axial direction of the transmission shaft 3.
The first and second cavities 21 and 22 are the same in structure. Therefore, the description thereof will be given below only of the first cavity 21.
In the first cavity 21, there are disposed a pair of trunnions 14. On the two end portions of each of the trunnions 14, pivot shafts 16 are disposed so as to be concentric with each other. These pivot shafts 16 are supported on the one-end portions of the pair of yokes 13a and 13b in such a manner that they can be swung and can be shifted in the axial direction thereof. That is, the pivot shafts 16 are supported inside support holes 18 formed in the one-end portions of the pair of yokes 13a and 13b by radial needle roller bearings 26 respectively. Each of the radial needle roller bearings 26 is composed of an outer race 27, the outer peripheral surface thereof has a spherical-shaped convex surface and the inner peripheral surface thereof has a cylindrical-shaped surface, and a plurality of needle rollers 28.
There is circular hole 30 on an intermediate of the respective trunnions 14. In the respective circular holes 30, shift shafts 31 are supported. Each of the shift shafts 31 includes a support shaft portion 33 and a pivot support shaft portion 34 which are parallel to each other but are eccentric with respect to each other. The support shaft portion 33 is supported inside the circular hole 30 through a radial needle roller bearing 35. On the periphery of the pivot support shaft portion 34, there is supported a power roller 36 through another radial needle roller bearing 38.
By the way, a pair of the shift shafts 31 are provided in every set of first and second cavities 21, 22. The pair of the shift shafts 31 are disposed in such a manner that they are situated on the 180° opposite side to the input shaft 2 and transmission shaft 3 respectively in every set of first and second cavities 21, 22. Also, the directions of the pivot support shaft portions 34 of the shift shafts 31 are eccentric with respect to the support shaft portions 33 of the shift shafts 31. The direction of the pivot support portion 34 is the same direction as the rotation direction of the first and second input disks 4, 5 as well as first and second output disks 7, 8. Further, the eccentric direction is also substantially perpendicular to the mounting direction of the input shaft 2. Therefore, each power roller 36 is supported in such a manner that it can be slightly shifted along the longitudinal direction of the input shaft 2 and transmission shaft 3. As a result of this, there is a case where the power roller 36 tends to shift in the axial direction of the input shaft 2 and transmission shaft 3 due to variations in the elastic deformation amount of component members caused by variations in the torque that is transmitted from the toroidal-type continuously variable transmission. In this case, there can be avoided the possibility that an unreasonable force can be applied to the component members, and thus the shifting movement of the power roller 36 can be absorbed.
Also, between the outer peripheral surface of the power roller 36 and the inner peripheral surface of the middle portion of the trunnion 14, there are interposed a thrust ball bearing 39 and a thrust bearing 40 such as a sliding bearing or a needle roller bearing by turns from the outer surface of the power roller 36. The thrust ball bearing 39, while supporting a thrust-direction load to be applied to the power roller 36, allows the power roller 36 to rotate. Also, the thrust bearing 40, while supporting a thrust load to be applied to the outer race 41 of the thrust ball bearing 39 from the power roller 36, allows the pivot support shaft portion 34 and outer race 41 to be swung about the support shaft portion 33.
To one end portion of each of the trunnions 14, there is connected a drive rod 42. To the outer peripheral surface of the middle portion of the drive rod 42, there is fixed a drive piston 43. This drive piston 43 is fitted oil-tight into a drive cylinder 44. And, the drive piston 43 forms an actuator, which is used to shift the trunnion 14 in the axial direction thereof.
As shown in FIG. 15, between the input shaft 2 and first input disk 4, there is interposed a pressure device 45 of a loading cam type. This pressure device 45 includes a cam plate 46 and a plurality of rollers 48 and is arranged such that, due to the rotation of the input shaft 2, it can press the first input disk 4 toward the second input disk 5 and rotate the first input disk 4. In this case, the cam plate 46 is spline engaged with the middle portion of the input shaft 2. The cam plate 46 is also supported in such a manner that it is prevented from shifting in the axial direction of the input shaft 2. The cam plate 46 can be rotated together with the input shaft 2. Also, the plurality of rollers 48 are rollably held on a retainer 47.
When the above toroidal-type continuously variable transmission is in operation, the rotation of the input shaft 2 is transmitted through the pressure device 45 to the first input disk 4, so that the first and second input disks 4 and 5 are rotated in synchronization with each other. The rotational movements of the first and second input disks 4 and 5 are transmitted through the power rollers 36 to the first and second output disks 7 and 8. The rotational movements of the first and second output disks 7 and 8 are taken out by the output gear 10.
To change a rotation speed ratio between the input shaft 2 and output gear 10, in accordance with the switching operation of a control valve (not shown), the drive pistons 43, a pair of which are disposed in each of the first and second cavities 21 and 22, may be shifted by the same distance in the mutually opposite directions in every cavities 21 and 22. With the shifting movements of the drive pistons 43, two pairs of trunnions 14, that is, a total of four trunnions 14 are respectively shifted in the opposite directions, so that one power roller 36 is shifted downward and the other power roller 36 is shifted upward. This changes the direction of a tangential-direction force which acts on the contact portion between the peripheral surfaces of the respective power rollers 36 and the inner surfaces 4a, 5a of the first and second input disks 4, 5 and the inner surfaces 7a, 8a of the first and second output disks 7, 8. In addition, with such change in the direction of the tangential-direction force, the trunnions 14 are swung in the opposite directions about the pivot shafts 16 pivotally supported on the yokes 13a and 13b. This changes the contact positions between the peripheral surfaces of the power rollers 36 and the first and second input disks 4, 5 as well as first and second output disks 7, 8, thereby changing the rotation speed ratio between the input shaft 2 and output gear 10.
However, in the above-structured conventional toroidal-type continuously variable transmission, since the trunnions 14 are supported inside the casing 1 through the support posts 20a, 20b and yokes 13a, 13b, the number of parts is increased. This complicates the manufacturing operation of the parts, the managing operation of the parts and the assembling operation of the parts. This also increases the height dimension of the toroidal-type continuously variable transmission to thereby be unable to reduce the size and weight of the toroidal-type continuously variable transmission.
Also, generally, in the case of a vehicle of an FR system, in order to be able to secure living space within the vehicle, the upper portion of the casing 1 must be formed compact. That is, as shown in FIG. 17, when the casing 1 is viewed from the axial direction thereof, it provides a projecting shape; and, the inside space of the casing upper portion 1a is formed narrower than the inside space of the casing lower portion 1b. 
In the above-mentioned conventional toroidal-type continuously variable transmission, the upper and lower portions of the trunnions 14 are swingably supported inside the casing 1 through the support posts 20a, 20b and yokes 13a, 13b. Then, in order to the upper yoke 13a can be swung about the support post 20a in the gear change operation, a sufficient inside space must be secured in the casing upper portion 1a. However, as described above, in case where the upper portion of the casing 1 must be formed compact, a sufficient inside space cannot be secured in the casing upper portion 1a. 
Also, there is a case where the upper and lower pivot shafts 16 of the trunnions 14 are swingably supported inside the casing 1 through the support posts 20a, 20b and yokes 13a, 13b. In this case, the number of parts increases, and it complicates that the operation to manufacture the parts, the operation to manage the parts and the operation to assemble the parts.
Further, the upper yoke 13a is connected to the upper pivot shaft 16 of the trunnion 14 through the radial needle roller bearing 26 restricted by an axial-direction restrict member which is designated by a numeral 49 in FIG. 17. However, in this structure, when the upper pivot shaft 16 of the trunnion 14 is swung, stresses are concentrated on the portions thereof which are contacted with the axial-direction restrict member 49 and radial needle roller bearing 26, which degrades the durability of the pivot shaft 16.
In view of the above, for example, as shown in FIG. 4 of Japanese Patent Unexamined Publication No. 2000-9200, there has been developed a structure in which a yoke is fixed directly to the inside of a casing. In addition there has been also developed two pivot shafts disposed on the two end portions of each trunnion are supported on the yoke through ball splines in such a manner that they can be moved in the vertical direction.
According to the above structure, since the yoke is fixed directly to the casing, the number of parts can be reduced. This can simplify the parts manufacturing operation, the parts managing operation and the parts assembling operation. In addition, this can also decrease the height dimension of the toroidal-type continuously variable transmission to thereby able to reduce the size and weight of the toroidal-type continuously variable transmission.
However, in the above-mentioned toroidal-type continuously variable transmission disclosed in Japanese Patent Unexamined Publication 2000-9200, the ball spline is formed in the pivot shaft of the trunnion and thus the pivot shaft is able to move in the vertical direction. Also, the outer race of the ball spline provides a spherical surface, whereby, when the trunnion is elastically deformed, the trunnion can be prevented from application of the edge loads. Further, due to the needle roller bearing of the inner race of the ball spline, the trunnion is able to rotate inclinedly about the pivot shaft.
Therefore, the trunnion support structure is complicated and thus the number of parts is also large. Also, when a ball is assembled into the upper ball spline, a hole must be made in the casing, which lowers the rigidity of the casing. Further, in the case of the ball spline which allows the trunnion to move in the vertical direction, as the ball movements, there coexist a sliding movement and a rolling movement according to the positions of the ball when assembling the ball into the upper ball spline. Generally, the friction coefficient of the rolling movement is at least one digit smaller than that of the sliding movement; and, for this reason, in case where a sliding movement exists together with the vertical-direction movements of the trunnions, the vertical-direction forces in the respective trunnions are uneven.
Also, according to the above-mentioned toroidal-type continuously variable transmission disclosed in Japanese Patent unexamined Publication 2000-9200, by fixing all yokes directly to the inside of the casing, the number of parts is reduced, which makes it unnecessary to secure the inner space of the casing. However, there is no system which, in the gear change operation, can mechanically guarantee the synchronization of the vertical-direction movements of all trunnions, so that the vertical-direction movements of the trunnions are unstable.